Both pairwise recombinational exchange between small plasmids bearing IS2 or IS3 and insertional activities of IS2 or IS3 will be studied to determine whether the preference for F integration at IS2 is related to any differences in the insertional activities at IS2 or IS3. Specific tra plus and tra minus F' plasmids will be characterized physically to determine the positions of the loci at which cross-over occurs on the bacterial DNA. Specifically, one of the EcoRI fragments containing a crossover point active in tra plus F' formation also contains IS1. This will be mapped to determine whether it coincides with the crossover point. For the tra minus F' plasmids, the bacterial DNA in which cross-overs occurred leading to different tra delta F' classes will be probed for homologies, and refined estimates of the degree of specificity will be made.